The invention relates to a bearing device for an exhaust gas turbocharger with a first radial bearing having a first bearing axis and a second radial bearing having a second bearing axis which extends coaxially with the first bearing axis as well as to an exhaust gas turbocharger.
A principal problem with a shaft is the bending of the shaft both in the static condition, i.e. without rotation of the shaft, and in the dynamic condition, i.e. during rotation, because of its weight and, in the case of rotation, due to an imbalance. In exhaust gas turbochargers, the shaft is usually rotatably supported at two end portions of the shaft by means of radial bearings. With small exhaust gas turbochargers as they are in particular employed in passenger car engines, bending is generally moderate due to the low weight of the shaft. However, these exhaust gas turbochargers exhibit a high operating speed of far beyond 150,000 rpm so that, even with a minute imbalance, circulating vibrations or bending vibrations, respectively, are generated. Under unfavorable conditions therefore a contact between the shaft and a casing portion may take place. Additionally, oil-induced low-frequency vibrations occur which are also referred to as oil whirl or oil whip. Firstly, this leads to noise emission, and secondly to a reduction of the service life of the radial bearings and thus to a reduction of the service life of the exhaust gas turbocharger.
Patent publication DE 36 01 082 C2 discloses an exhaust gas turbocharger whose rotating assembly comprising a compressor wheel, a turbine wheel as well as a shaft, which non-rotatably connects the compressor wheel with the turbine wheel, is supported by means of a plain bearing device in a bearing portion of the exhaust gas turbocharger. The bearing device comprises a bearing bushing which virtually extends from one end of the shaft to the other end of the shaft, wherein a radial bearing each in the form of a plain bearing is provided in the end areas. To prevent the bearing bushing from being rotated, anti-rotation means are formed. In order to enable the lubricant supplied to the bearing device to be reliably discharged for preventing a so-called oil whirl or oil whip, respectively, recesses are formed in the bearing bushing between the radial bearings. A reliable rigidity of the bearing bushing is ensured by means of two lands which are formed in the area of the recesses. The manufacture of such a bearing bushing however is expensive due the lands and recesses.
It is the object of the present invention to provide a bearing device for an exhaust gas turbocharger by means of which a quasi-static support of a shaft of the exhaust gas turbocharger can be achieved. Another aspect of the invention is to provide an exhaust gas turbocharger with an improved bearing device.